The polar modulation transmitter is a type of radio frequency (RF) transmitter commonly used in wireless communications. The principal elements of a polar modulation transmitter 100 are shown in FIG. 1. They include a baseband processor 102; a COordinate Rotation DIgital Computer converter (CORDIC) 104; an amplitude path digital-to-analog converter (DAC) 106 and an envelope modulator 108 in an amplitude path; a phase path DAC 110 and an RF oscillator 112 in a phase path; a power amplifier (PA) 114; and an antenna 116.
During operation, the baseband processor 102 generates digital in-phase (I) and quadrature phase (Q) signals from a digital message to be transmitted. The CORDIC converter 104 converts the rectangular-coordinate digital I and Q into polar-coordinate digital envelope and phase component signals ρ and θ. The envelope and phase path DACs 106 and 110 convert the digital envelope and phase component signals ρ and θ into analog envelope modulation and phase modulation signals. The envelope modulator 108 modulates a DC power supply 118 according to amplitude variations in the analog envelope modulation signal, thereby generating an amplitude modulated power supply signal. Meanwhile, the analog phase modulation signal in the phase path is used to modulate an RF carrier signal generated by the RF oscillator 112. The resulting phase modulated RF signal is coupled to an RF input of the PA 114, and the amplitude modulated power supply signal from the envelope modulator 108 is coupled to a power supply input of the PA 114.
Because the phase modulated RF signal has a constant envelope, the PA 114 can be configured to operate in its nonlinear region of operation, where it is much more efficient in converting DC power from the DC power supply 118 to RF power than it is when configured to operate in its linear region. Typically, the PA 114 is implemented as a Class D, E or F switch-mode PA 114 operating in compression, so that the output power of the PA 114 is directly controlled by the amplitude modulated power supply signal applied to the power supply input of the PA 114. Hence, the PA 114 effectively operates as an amplitude modulator, amplifying the constant-envelope phase modulated RF signal according to amplitude variations in the amplitude modulated power supply signal, to produce a PA output signal that is both amplitude and phase modulated.
Operating the PA 114 in its nonlinear region, where it is most efficient, is highly desirable, particularly in battery-powered applications where power efficiency is an overriding concern. However, when it is operated in its nonlinear region, the PA 114 undesirably distorts the signals it amplifies. One type of distortion, known as amplitude modulation to amplitude modulation (or “AM/AM”) distortion, results from the fact that the gain of the PA 114 compresses for higher values of input voltage (i.e., for higher amplitudes of the amplitude modulated power supply signal applied to the power supply input of the PA 114). This AM/AM distortion effect is illustrated in FIG. 2. Another type of distortion, known as amplitude modulation to phase modulation (or “AM/PM”) distortion, results from an undesirable phase modulation of the PA output signal by an out-of-phase signal leaked from the RF input of the PA 114 to the RF output of the PA 114. The degree of AM/PM distortion introduced into the output signal depends on the amplitude of the amplitude modulated power supply signal relative to the amplitude of the leaked signal. Generally, the larger the amplitude of the leaked signal is relative to the amplitude of the amplitude modulated power supply signal, the larger the amount of AM/PM distortion.
AM/AM and AM/PM distortion can make it difficult to comply with noise specifications set forth by wireless communications standards. Fortunately, various linearization techniques are available to negate or counteract the effects of AM/AM and AM/PM distortion, yet which still allow the PA 114 to be operated efficiently in its nonlinear region. One approach involves predistorting the signals in the amplitude and phase paths according to inverses of known AM/AM and AM/PM distortion responses of the PA 114, so that the applied predistortions counteract the AM/AM and AM/PM distortions caused by the PA 114. FIG. 3 illustrates, for example, how a signal is predistorted according to an inverse of a known AM/AM distortion response, so that AM/AM distortion caused by the PA 114 is counteracted. The amplitude response of the PA with the predistortion having been applied is seen to more closely resemble an ideal PA response compared to if no predistortion had been applied.
FIG. 4 is a diagram illustrating how the polar modulation transmitter 100 in FIG. 1 is typically modified to include the ability to apply AM/AM and AM/PM predistortion to the digital envelope and phase component signals ρ and θ. A controller 406 of the modified polar modulation transmitter 400 retrieves AM/AM and AM/PM predistortion coefficients from a memory 408, and provides the coefficients to AM/AM and AM/PM predistorters 402 and 404, which operate to predistort the digital envelope and phase component signals ρ and θ according to the retrieved AM/AM and AM/PM predistortion coefficients.
In a typical configuration, the AM/AM and AM/PM predistortion coefficients are stored in the memory 408 as a plurality of LUTs 502-1, . . . , 502-p (p is an integer greater than or equal to one), as illustrated in FIG. 5. The plurality of LUTs 502-1, . . . , 502-p corresponds to a plurality of average power levels P1, P2, . . . , Pp the PA 114 can be commanded to operate. Each LUT of the plurality of LUTs 502-1, . . . , 502-p includes a unique set of digital AM/AM predistortion coefficients a1(Px), a2(Px), . . . , al(Px) and a corresponding unique set of digital AM/PM predistortion coefficients p1(Px), p2(Px), . . . , pl(Px), where l is an integer greater than or equal to two and x={1, . . . , p}. The l different AM/AM and AM/PM predistortion coefficients in a selected LUT provide the ability to predistort the digital envelope and phase component signals ρ and θ at l different predistortion levels. Which particular LUT of the plurality of LUTs 502-1, . . . , 502-p is selected is determined by a transmit power level signal received by the controller 406 from the baseband processor 102. Once a particular LUT has been selected, the controller 406 retrieves AM/AM and AM/PM predistortion coefficients from the selected LUT according to the amplitudes represented in the digital values of the digital envelope component signal p.
The digital AM/AM and AM/PM predistortion coefficients a1 (Px), a2(Px), . . . , al(Px) and p1(Px), p2(Px), . . . , pl(Px) of the plurality of LUTs 502-1, . . . , 502-p are determined by performing a distortion characterization process on the PA 114, before the polar modulation transmitter 100 is put into service. The distortion characterization process includes measuring the AM/AM and AM/PM distortion behavior of the PA 114 at each of the different average power levels P1, P2, . . . , Pp, calculating the digital values of the digital AM/AM and AM/PM predistortion coefficients a1(Px), a2(Px), . . . , al(Px) and p1(Px), p2(Px), . . . , pl(Px) from the measured AM/AM and AM/PM distortion behavior for each of the average power levels P1, P2, . . . , Pp, and evaluating, at each of the average power levels P1, P2, . . . , Pp, the effectiveness of the calculated digital AM/AM and AM/PM predistortion coefficients a1(Px), a2(Px), . . . , al(Px) and p1(Px), p2(Px), . . . , pl(Px) at countering the AM/AM and AM/PM distortion caused by the PA 114. If the polar modulation transmitter 400 is operable over multiple frequency bands, then further characterization is performed for each of the frequency bands and again for each of the average power levels P1, P2, . . . , Pp. As will be appreciated by those of ordinary skill in the art, this distortion characterization process can be very time consuming, particularly when there are a large number p of power levels and frequency bands involved.
The distortion characterization process is not only time-consuming, but the amount of memory needed to store the plurality of LUTs 502-1, . . . , 502-p can be quite large. In most modern-day polar modulation transmitter architectures, the memory 408 is formed on the same integrated circuit (IC) chip as other elements of the transmitter 400. To economize the design and leave room for the other elements, it is, therefore, important that the LUTs 502-1, . . . , 502-p occupy as little of the memory 408 as possible. Unfortunately, this may be difficult or even impossible to achieve. In a Global System for Mobile Communications (GSM)/Enhanced Data Rate for GSM Evolution (EDGE) cellular communications application, for example, predistortion coefficients are determined from a characterization of the PA 114 at sixteen (16) different average power levels and then repeated for each GSM/EDGE frequency band. In a Wideband Code Division Multiple Access (W-CDMA) application, the PA 114 can be commanded to operate at up to eighty (80) different average power levels. The required memory space would, therefore, have to be at least five times larger than in a GSM/EDGE application. In a design originally targeted at a GSM/EDGE application, the memory space needed to accommodate the predistortion LUTs for each of these eighty different power levels would likely not be available. In order to accommodate all of the predistortion coefficients, therefore, the IC would have to be redesigned to include the needed additional memory.
In addition to time-consuming characterization and limitations on available memory, the predistortion coefficients of the plurality of LUTs 502-1, . . . , 502-p must be downloaded into the memory 408 (e.g., from an external FLASH memory device) each time power is cycled. The more memory the plurality of LUTs 502-1, . . . , 502-p occupy, the longer the download time is. If the download time is excessive, it can be an annoyance to users of the communications device in which the polar modulation transmitter 400 is being used.
It would be desirable, therefore, to have methods and apparatus for predistorting communications signals that do not require time consuming characterization processes and which do not require large amounts of memory to store predistortion LUTs.